1. Field of the Invention
This invention relates generally to pressure sensors, and more particularly, to integrated circuit pressure transducers having on chip circuitry that compensates for voltage shifts in the output of the pressure transducer due to changes in temperature.
2. Background Art
Numerous medical and industrial applications require an electronic pressure measurement that is relatively inexpensive, accurate over a limited temperature range, and requires little space. One such application involves measuring a person's blood pressure by placing a small pressure transducer in a saline solution that is connected intravenously to the person's circulatory system. The output of the pressure transducer is connected to an apparatus that displays the blood pressure visually.
Unfortunately, most semiconductor pressure transducers have undesirable temperature characteristics which are different for different transducers. Offset voltage, which is the differential output voltage of the transducers at zero pressure, not only varies in magnitude for different transducers, but also varies with temperature change. Also, the temperature coefficient of "span", or change in voltage output versus change in pressure, typically has a negative value, the range of values varying for both different transducers and changes in temperature.
One known configuration for temperature compensation of span is a temperature compensation circuit that utilizes a plurality of thermistors which vary the magnitude of the excitation voltage across the transducer to compensate for the undesirable changes in sensitivity with temperature. The pressure transducer is basically a bridge circuit and the thermistors are connected from each input terminal of the bridge to a power supply line. The thermistors change the excitation voltage level so that the output voltage across the terminals of the bridge remain constant for a given change in pressure even though the temperature changes. The thermistors have been shunted with temperature stable elements such as resistors to tailor the compensation characteristic. The combination of resistors, thermistors, and transducer has been adjusted by laser trimming through iterative operations over temperature to provide a composite device having a desired degree of temperature independence. These adjustments include sequential measurements over temperature and trimming. These elements can also be trimmed to compensate for the undesired temperature dependence of the transducer offset voltage.
Another known configuration requires the insertion of jumpers or connecting wires to complete parts of the circuit. The circuit must be tested with each individual transducer to determine whether the temperature coefficient of offset is positive or negative and then a jumper inserted to have the circuit compensate appropriately. The inclusion of jumpers results in a major cost increase. Furthermore, the procedure for inserting the jumpers induces inaccuracies into the circuit. The jumper insertion device picks up radio frequency interference from the laser used for trimming the resistors and typically induces up to 30 millivolts into the output of the circuit.
Yet another known configuration utilizes a compensating means having two thermistors. The thermistors are coupled to a high and low side of the voltage source, respectively, and each provide a signal to the excitation terminal of the pressure transducer. The pressure transducer output is summed with the signals from both thermistors, the summation thereafter being amplified.
The above configurations require negative temperature coefficient thermistors which cannot be placed on a chip of an integrated circuit. Furthermore, the known configurations may require a complex procedure for trimming the resistors. The gain of the circuit and the offset voltage of the transducer interact, requiring different pressures for setting both. Thus, the need exists for a temperature compensating pressure transducer circuit that, along with the pressure transducer, is an integrated circuit that does not use negative coefficient thermistors, does not have jumpers and wherein gain and transducer offset do not interact thereby simplifying the resistor trimming procedure.